Cambridge AS/A Level Biology

Defintions of AS/A Level Biology Key Words

cell

the basic unit of all living organisms; it is surrounded by a cell surface membrane and contains genetic material (DNA) and cytoplasm containing organelles

organelle

a functionally and structurally distnct part of a cell, e.g. a ribosome or mitochondrion

nucleus (plural: nuclei)

a relatively large organelle found in eukaryotic cells, but absent from prokaryotic cells; the nucleus contains the cell’s DNA and therefore controls the activities of the cell; it is surrounded by two membranes which together form the nuclear envelope

eukaryote

an organism whose cells contain a nucleus and other membrane-bound organelles

prokaryote

an organism whose cells do not contain a nucleus or any other membrane-bound organelles

cell surface membrane

a very thin membrane (about 7 nm diameter) surrounding all cells; it is partially permeable and controls the exchange of materials between the cell and its environment

chromatin

the material of which chromosomes are made, consisting of DNA, proteins, and small amounts of RNA; visible as patches or fibres whithin the nucleus when stained

chromosome

in the nucleus of the cells of eukaryotes, a structure made of tightly coiled chromatin (DNA, proteins and RNA) visible during cell division; the term ‘circular DNA’ is now also commonly used for the circular strand of DNA present in a prokaryotic cell

nucleolus

a small structure, one or more of which is found inside the nucleus; the nucleolus is usually visible as densely stained body; its function is to manufacture ribosomes using the information in its own DNA

protoplasm

all the living material inside a cell (cytoplasm plus nucleus)

cytoplasm

the contents of a cell, excluding the nucleus

mitochondrion (plural: mitochondria)

the organelle in eukaryotes in which aerobic respiration takes place

cell wall

a wall surrounding prokaryote, plant and fungal cells, the wall contains a strengthening material which protects the cell from mechanical damage, supports it and prevents it from busting by osmosis if the cell is surrounded by a solution with a higher water potential

plasmodesma (plural: plasmodesmata)

a pore-like structure found in plant cell walls; plasmodesmata or neighbouring plant cells line up to form tube-like pores through the cell walls, allowing the controlled passage of materials from one cell to the other; the pores contain ER and are lined with the cell surface membrane

vacuole

an organelle found in eukaryotic cells; a large, permanent central vacuole is a typical feature of plant cells, where it has a variety of functions, including storage of biochemicals such as salts, sugars and waste products; temporary vacuoles , such as phagocytic vacuoles (also known as phagocytic vesicles), may form in animal cells

tonoplast

the partially permeable membrane that surrounds plant vacuoles

chloroplast

an organelle, bounded by an envelope (i.e. two membranes), in which photosynthesis takes place in eukaryotes

photosynthesis

the production of organic substances from inorganic ones, using energy from light

grana (singular: granum)

stacks of membranes inside a chloroplast

magnification

the number of times larger an image of an object is than the real size of the object; magnification equals to the image size divided by the actual(real) size of the object (image size/actual size)

eyepiece graticule

small scale that is placed in a microscope eyepiece

stage micrometer

very small, accurately drawn scale of known dimensions, engraved on a microscope slide

micrograph

a picture taken with the aid of a microscope; a photomicrograph (or light micrograph) is taken using a light microscope; an electron micrograph is taken using an electron microscope

resolution

the ability to distinguish between two objects very close together; the higher the resolution of an image, the greater the detail that can be seen

microvilli (singular: microvillus)

small, finger-like extensions of a cell which increase the surface area of the cell for more efficient absorption or secretion

nuclear envelope

the two membranes, situated close together, that surround the nucleus; the envelope is perforated with nuclear pores

nuclear pores

pores found in the buclear envelope which control the exchange of materials, e.g. mRNA, betweeen the nucleus and the cutoplasme

endoplasmic reticulum (ER)

a network of flattened sacs running through the cytoplasm of eukaryotic cells; molecules, particularly proteins, can be transported through the cell inside the sacs separate from the rest of the cytoplasm; ER is continuous with the outer membrane of the nuclear envelope

ribosome

a tiny organelle found in large numbers in all cells; prokaryotic ribosomes are about 20 nm in diamter while eukaryotic ribosomes are about 25 nm in diamter

Golgi apparatus (Golgi body, Golgi complex)

an organelle found in eukaryotic ells, the Golgi apparatus consists of a stack of flattened sacs, constantly forming at one end and breaking up in to Golgi vesicles at the other end

Golgi vesicles

carry their contents to other parts of the cell, often to the cell surface membrane for secretion; the Golgi apparatus chemically modifies the molecules it transports, e.g. sugars may be added to proteins to make glycoproteins

lysosome

a spherical organelle found in eukaryotic cells; it contains digestive (hydrolytic) enzymes and has a variety of destructive functions, such as removal of old cell organelles

cristae (singular: crista)

folds of the inner membrane of the mitochondrial envelope on which are found stalked particles of ATP synthase and electron transport chains associated with aerobic respiration

ATP (adenosine triphosphate)

the molecule that is the universal energy currency in all living cells; the purpose of respiration is to make ATP

ADP (adenosine diphosphate)

the molecule that is converted to ATP by addition of phosphate (a reaction known as phosphorylation) during cell respiration; the enzyme responsible is ATP synthase; the reaction requires energy

microtubes

tiny tubes made of a protein called tubulin and found in most eukaryotic cells; microtubules have a large variety of functions, including cell support and determining cell shape; the ‘spindle’ on which chromatids and chromosomes separate during nuclear division is made of microtubules

centriole

one of two small, cylindrical structures made from microtubules, found just outside the nucleus in animal cells, in a region known as the centrosome; they are also found at the bases of cilia and flagella

centrosome

the main microtubule organising centre (MTOC) in animal cells

cilia (singular: cilium)

whip-like structures projecting from the surface of many animal cells and the cells of many unicellular organisms; they beat, causing locomotion or the movement of fluid across the cell surface

flagella (singular: flagellum)

whip-like structures projecting from the surface of some animal cells and the cells of many unicellular organisms; they beat, causing locomotion or the movement of fluid across the cell surface; they are identical in structure to cilia, but longer

thylakoid

a flattened, membrane-bound, fluid-filled sac which is the site of the light-dependent reactions of photosynthesis in a chloroplast

bacteria (singular: bacterium)

a group of single-celled prokaryotic microorganisms; they have a number of characteristics, such as the ability to form spores, which distinguish them from the other group of prokaryotes known as Archaea

peptidoglycan

a polysaccharide combined with amino acids; it is also known as murein; it makes the bacterial cell wall more rigid

plasmid

a small circular piece of DNA in a bacterium (not its main chromosome); plasmids often contain genes that provide resistance to antibiotics

virus

a very small (20-300 nm) infectious particle which can replicate only inside living cells; it consists of a molecule of DNA or RNA (the genome) surrounded by a protein coat; an outer lipid envelope may also be present

phospholipid

a lipid to which phosphate is added; the molecule is made up of a glycerol molecule, two fatty acids and a phosphate group; a double layer (a bilayer) of phospholipids forms the basic structure of all cell membranes

macromolecule

a large molecule such as a polysaccharide, protein or nucleic acid

polymer

a giant molecule made from many similar repeating subunits joined together in a chain; the subunits are much smaller and simpler molecules known as monomers; examples of biological polymers are polysaccharides, proteins and nucleic acids

momnomer

a relatively simple molecule which is used a basic building block for the synthesis of a polymer; many monomers are joined together by covalent bonds to make the polymer, usually by condensation reactions; common examples of monomers are monosaccharides, amino acids and nucleotides

condensation reaction

a chemical reaction involving the joining together of two molecules by of a water molecule

hydrolysis

a chemical reaction in which a chemical bond is broken by the addition of a water molecule; commonly used to break down complex molecules into simpler molecules

monosaccharide

a molecule consisting of a single sugar unit and with the general formula (CH2O)n

disaccharide

a sugar molecule conssting of two monosaccharides joined together by a glycosidic bond

glycosidic bond